1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus provided with the same, and particularly relates to a removal of slackness of a web (corresponding to a roll of papers, actually a non-woven fabric is used) for supplying a mold release onto a surface of a fixing roller.
2. Description of the Prior Art
In a conventional fixing apparatus of an image forming apparatus using an electrophotographic system, there are employed various kinds of techniques for applying a mold release such as a silicone oil and the like to a fixing roll so as to prevent a toner from offsetting. Particularly, in a full color image forming apparatus in which a plurality of different color toners are mixed, since the fixing roll requires a very high mold release property, a large amount of mold release is applied to the fixing roll.
Here, in the conventional technique, the mold release as been mainly supplied by a structure in which a roll and he like impregnated with the mold release such as a silicone oil and the like is directly brought into contact with the surface of the fixing roll. However, in the impregnated roll system, a supply amount of the mold release is varied in accordance with a change with the passage of time, so that there is a problem that an offset effect is not maintained for a long time.
Then, in order to solve this problem, there has been suggested a sheet type mold release supplying structure which takes up a web integrated with a mold release while bringing the web into contact with the fixing roll.
Next, an image forming apparatus in which a fixing apparatus in accordance with a conventional sheet type mold release supplying structure is employed will be explained below.
A development unit for receiving toners respectively corresponding to black (B), cyan (C), magenta (M) and yellow (Y), a charging unit for charging a photosensitive body belt due to a corona discharge and the like are provided along an outer peripheral surface of the photosensitive body belt which is coated with a photosensitive image receiving layer and circulates. Further, there is provided a laser scanning unit (an LSU) incorporating a semiconductor laser element therein and irradiating a laser beam onto the photosensitive body belt while turning on and off the laser beam in correspondence to the image data so as to form an electrostatic latent image. Still further, in opposition to the photosensitive body belt, an intermediate transferring body belt on which the toner image on the photosensitive body belt is transferred is placed.
Then, a paper feeding path is formed in such a manner as to be brought into contact with the circulating intermediate transferring body belt, and the toner image formed on the intermediate transferring body belt in an overlapping manner is transferred to the paper conveyed on the paper feeding path.
A fixing apparatus is placed in a downstream side of the paper feeding path, and an unfixed toner image transferred to the paper is heated and pressure fixed by the fixing apparatus.
Here, a structure of the fixing apparatus for fixing the toner image to the paper will be described below.
FIG. 6 is a schematic view which shows a structure of the fixing apparatus in a conventional image forming apparatus.
The fixing apparatus has a fixing roller 8 heated so that a temperature of a surface becomes an optimum fixing temperature (160.degree. C. to 180.degree. C.) by an internal or external heat source, a pressurizing roller 9 for holding the conveyed unfixed toner image therebetween together with the paper, and a web unit 10 corresponding to mold release applying means for applying a silicone oil as a mold release to the fixing roller 8 and cleaning the surface of the fixing roller 8 finishing a fixing operation.
The web unit 10 is constituted by a first shaft 12a formed by a sheet-like member made of a non woven fabric and a porous material and around which a web 11 impregnated with the mold release such as a silicone oil is wound, a second shaft 12b applying the mold release onto a surface of the fixing roller 8 and taking up the used web 11 having a surface cleaned, and a friction plate 13 brought into contact with the unused web 11 wound around the first shaft 12a, that is, an outer peripheral surface of the web roll by means of urging means (not shown) such as a spring and the like. Then, the web 11 is brought into contact with the surface of the fixing roller 8 by a pressing roller 14 from a side of a back surface.
In this case, the fixing roller 8 is structured such that an elastic layer 16 made of a high temperature vulcanized silicone rubber (an HTV silicone rubber) is formed on a core bar 15 made of an aluminum, an oil resistant layer F made of a fluoro rubber is formed on the elastic layer 16, and an offset prevention layer 17 made of a regular temperature vulcanized silicone rubber (an RTV silicone rubber) is further formed thereon. In this case, an outer diameter of the fixing roller 8 is, for example, set to 40 mm.
The pressing roller 9 is structured such that an elastic layer 19 made of the HTV silicone rubber is formed on a core bar 18 made of an aluminum, and a resin layer 20 made of a fluoro resin is formed on the elastic layer 19. An outer diameter of the pressing roller 9 is, for example, set to 40 mm.
A halogen heater 21 is arranged within each of the core bar 15 of the fixing roller 8a and the core bar 18 of the pressing roller 9. Then, it is structured such that a temperature of the fixing roller 8 is detected by a thermistor 22 brought into contact with the fixing roller 8 so as to turn on and off the halogen heater 21, thereby constantly maintaining the temperature of the fixing roller 8 to about 165.degree. C.
The web unit 10 is structured so as to press the web 11 impregnated with the silicone oil having a viscosity 500 cS to the fixing roller 8 by the pressing roller 14 by uniformly feeding the web 11 through a drive gear 25A of a drive mechanism 25 so as to be wound around the second shaft 12b, thereby applying the web 11 to the offset prevention layer 17 of the fixing roller 8, and at the same time of this, a toner offset (attached) to the offset prevention layer 17 of the fixing roller 8 is removed. Further, the friction plate 13 pressure contacted on the outer periphery of the web roll generates a friction force in the web roll at a time when the web 11 is taken up due to the pressing force, and applies a proper tension to the web 11 interlinked between the first shaft 12a and the second shaft 12b.
In this case, the web unit 10 is detachably mounted to the main body of the fixing apparatus, and is structured so as to be replaced when fully using the web 11.
In accordance with the web unit 10 structured in the above manner, the layered toner image can be thermally fixed on the paper in a mixed manner by passing the paper carrying a plurality of unfixed toner images in a layered manner between the rotating fixing roller 8 and pressing roller 9 in a gripping manner, so that a full color image can be formed.
However, in accordance with the conventional technique mentioned above, even in the case that a slackness is generated in the web 11 at a time of replacing the web unit 10 and manually rotating the fixing roller 8 in an inverse direction for removing a paper in accordance with a jamming operation, the slackness can not be removed and a printing operation is performed in a loosened state. Further, when feeding the web 11 in a state that no tension exists in the web 11, rumples are generated in the web 11 at a pressure contact portion with respect to the pressing roller 14, so that an adhesion property to the surface of the fixing roller 8 is deteriorated. Accordingly, an application unevenness of the silicone oil corresponding to the mold release is generated, so that the toner can be easily attached to the surface of the fixing roller 8. Therefore, a generating rate of the offset is increased and a cleaning performance of the surface of the fixing roller 8 is decreased.
Further, when driving the fixing roller 8 in a state that the web 11 is loosened, a contact area between the web 11 and the fixing roller 8 becomes wider than that of a normal state due to a slackness of the web 11, so that a friction load due to the fixing roller 8 is increased. Accordingly, there has been a problem that the web 11 taken up in the second shaft 12b is inversely drawn due to an excess friction load and is inversely returned.